Interferometers are widely used for measuring the profiles of smooth surfaces. In these devices, interfering beams are reflected from different areas on a solid surface and interferometric techniques are used to detect variations in surface height by measuring the path differences between the two interfering beams as the beams are moved across the solid surface. The interferometry transforms phase information encoded on a beam of coherent light into an intensity modulation that can be detected by conventional "square-law" detectors. The encoded information is typically a representation of fringes, in time or space, whose repeat frequency is determined by the period of one cycle of the optical wavelength. The fringe separation can be scaled somewhat by the geometry of the apparatus, but in general one fringe cannot be distinguished from another. As a result, it is not practical to identify a single fringe uniquely. This problem can be partially remedied when the bandwidth of the light is selected to give the highest contrast in the "zero-order" fringe, and which the contrast is reducing in either direction. If the contrast of the fringe reduces rapidly enough to give unambiguous identification, the distance over which a useful measurement can be discriminated is proportionally altered (i.e. reduced or increased).
In addition to the fringe detection problems, typical interferometers are notoriously difficult to keep stable. Since the optical paths of light in different routes are the source of the desired measurement, any differences arising from effects other than that to be measured are ruinous to the accuracy. These include mechanical vibration; thermal expansion; intensity modulation; birefringence effects; spatial and temporal coherence reduction; optical, mechanical and/or electronic drift; and/or other unanticipated operational problems.
Previous interferometers have been able to measure relative surface displacements over a limited range. This has made it impossible to contour surfaces containing steps or other discontinuities. Previous interferometers have also used bulk optics for guiding and mixing the light. Bulk optics suffer from alignment, stability and fringe contrast problems.
In view of the deficiencies of present interferometers used to measure relative surface displacement, there is a need for an interferometer that is simple to use, overcomes past stability and fringe contrast problems, and can measure relative surface displacements over a larger range than heretofore.